This invention is directed to a heat absorbing, neutral gray colored glass composition. More particularly, it comprises soda-lime-silica glass whose coloring components consist essentially of iron oxide, cobalt, selenium, manganese dioxide, and optionally titanium dioxide.
Glass having a neutral gray color and properties such as low infra red transmittance and low total solar energy transmittance which reduce the heat gain in the interior of an enclosure is highly desired in the automotive field. The neutral gray color is chosen for the sake of coordinating with a wide range of automotive paint colors. To be particularly useful in the automotive field, the glass composition should be one that is compatible with flat glass manufacturing methods. A glass composition having these properties would be highly desirable, particularly for automotive privacy and sun roof applications.
Some other heat absorbing gray glass compositions contain selenium as an essential coloring component. For example, U.S. Pat. No. 4,873,206 to Jones discloses a gray glass composition which includes as the colorants only iron, cobalt, and selenium. Selenium is a relatively low melting component with a melting point of 217.degree. C. and a boiling point of 685.degree. C. which typically leads to a volatilization of 85% or more of the selenium from the glass batch during glass melting and processing. Selenium is a very expensive material and hence its emission during glass melting is less than desirable. This high level of volatilization takes place even though glass manufacturers typically use sodium or potassium nitrates in the glass composition in an attempt to retain more of the selenium in the glass product.
The present invention overcomes problems associated with selenium vaporization by incorporating manganese oxide into a grey glass composition along with the selenium. We have unexpectedly found that incorporating the manganese oxide into a selenium containing glass composition aids in selenium retention during melt processing.
In addition, contrary to the suggestion of Jones in the patent listed above, including manganese oxide in the present invention composition along with the iron oxide did not lead to increased solarization. Solarization is the result of reactions that occur in glass when exposed to UV radiation such as that in sunlight. One such reaction is the shift of Fe.sup.+3 towards Fe.sup.+2. Solarization causes the iron to move from the oxidized species to the reduced species which also causes an undesirable color shift in the glass product. This problem of solarization is also discussed by Milos B. Volf in Chemical Approach to Glass, Elsevier Science Publishing Co., Inc., 1984, p. 343. "Manganese as a decolorizer has the disadvantage of creating solarization and of being sensitive to the furnace atmosphere."
Generally, gray colored heat absorbing glasses relied, in the past, on the inclusion of nickel oxide as an active coloring agent. Nickel compounds, however, are known to react with other materials in soda-lime-silica glass and form nickel sulfide "stones" in the glass. These stones are usually small, thereby avoiding detection methods, but can produce an unacceptably high rate of breakage during tempering of the glass. U.S. Pat. No. 5,023,210 to Krumwiede et al. discloses the use of chrome oxide in combination with iron oxide, cobalt oxide and selenium to achieve a dark gray glass without nickel. It does not include MnO.sub.2.